Cement panels consisting predominantly of asbestos fibers and cement and thus reinforced with the fibers are widely used as roof and wall materials for buildings because they are flame-retardant and outstanding in mechanical strength, heat insulating properties and soundproofness.
Cement panels reinforced with fibers include those produced by a dry method in which water is applied to a layer composed predominently of a mixture of asbestos fibers and cement, and the wet layer is pressed and thereafter shaped. The cement panels obtained contain a reduced amount of water, have relatively high freeze-thaw resistance and are therefore advantageous to use in a cold climate. If asbestos-cement panels contain residual water, the water freezes and expands when it is cold, and the frozen water contracts on thawing. Consequently, daily variations in the atmospheric temperature of cold climate produces stress in the asbestos-cement panel due to its expansion and contraction. When the panel is repeatedly subjected to such stress, cracks will eventually develop in the panel. The higher the water content, the greater is the stress and the greater will be the susceptibility of the panel to cracking. Thus the use of asbestos-cement panels made by the dry method and having a lower residual water content is advantageous in avoiding the cracking, namely the damage due to freezing and thawing.
As shown in FIG. 4, conventional cement panels reinforced with fibers and produced by the dry method comprises a base plate 1 made predominantly from asbestos fibers and cement, a coloring material layer 2 composed predominantly of portland cement and pigment and bonded to the base plate 1 to give an improved appearance to the base plate 1, and a colorless transparent surface waterproof layer 3 covering the surface of the coloring material layer 2 and formed by applying a liquid acrylic resin composition to the surface.
However, the fiber-reinforced cement panels made by the dry method still involve the problem that when they are used as roof tiles for buildings in a cold district, the coloring material layer 2 becomes separated from the base plate 1 over an area of 1 to 2 cm in diameter.
When such fiber-reinforced cement panels were tested for freeze-thaw resistance in accordance with Method C 290-61 T specified by ASTM in which they were subjected to freeze-thaw cycles by being allowed to stand at -18.degree. C for 2 hours and at +5.degree. C for 1 hour alternately repeatedly, separation occurred when they were exposed to 60 cycles. Although the mechanism in which the freeze-thaw cycles cause the separation of the coloring material layer 2 still remains to be fully clarified, it is believed that the water migrating from the base plate 1 to the coloring material layer 2 repeatedly freezes and thaws, resulting in stress cycles and thereby breaking down the coloring material layer which has relatively low resistance.
Japanese Patent Publication SHO 46-1419 discloses a method in which a solidified base plate is covered with a layer of coloring cement material. However, since no consideration whatever is given to its freeze-thaw resistance, the covering layer appears very prone to separation when the residual water is subjected to freeze-thaw cycles.
In an attempt to improve the freeze-thaw resistance, I have carried out intensive research and accomplished the method of this invention for producing cement panels reinforced with fibers and having satisfactory properties.